Stabilizing molten material during magnetic levitation and heating thereof



Aug. 17, 1954 J, c. R. KELLY, JR 2,686,865

STABILIZING MOLTEN MATERIAL DURING MAGNETIC LEVITATION AND HEATINGTHEREOF Filed Oct. 20, 1951 f j 41,3 f9- O 47 V "L --Jf 41 15 16 I I 351 1 i; \\/11 45/ l j iz ,4g 51 l1? M 18 19 Z1 39 Z5 ZZ j wm/'afA/Efwrazzz Z7 .e5 z3 ijf-9.2. 1%@

\ ATORNEY Patented Aug. 17, 1954 STABILIZING MOLTEN MATERIAL DURINGMAGNETIC LEVITATION AND HEATING THEREOF John C. R. Kelly, Jr., U

signor to Westingho East Pittsburgh, Pa

Sylvania Application October 20, 1951,

7 Claims.

This invention relates to heating and/or melting of electricallyconductive materials in any form and shape while levitated by a properlydistributed alternatingI magnetic field or fields in air, vacuum orinert gas atmosphere, without a confining container or crucible. Therequired levitating field, which may be obtained by variousconfigurations of coils, is shielded from undesired distortion by thecurrent in the power supply leads. The frequency of the alternatingmagnetic field is preferably such that relatively thin skin depth offield penetration occurs in the conductive object.

The principal object of my invention, generally considered, is toprovide apparatus for heating conductive materials, comprising at leastone, but preferably a system of electrical circuits or coils, means forsupplying an alternating magnetic field or elds, whereby a conductiveobject interposed above a coil or between coils on the common axisthereof may be levitated thereat by the alternatingI current fields,means to prevent undesired distortion of the field or fields, and meansfor moving such an object into the influence of said fields whereby itmay be levitated and heated to the desired extent, out of contact with apotentially contaminating crucible or other container at the elevatedtemperature involved.

Other objects and advantages, relating to the particular arrangement andthe construction of the various parts, will become apparent as thedescription proceeds.

Referring to the scale drawings:

Figure l is a vertical sectional View of apparatus for producingalternating current fields, in air, a vacuum, or an inert atmosphere,for levitating conductive material to be melted and/or heated, means forshielding the fields from the effects of current in the leads or coilexmeans for raising supporting means for such material to a positionbetween two coaxial coil systems where the fields will exert the desiredlevitation, and then Withdrawing the supporting means immediately.

Figure 2 is a horizontal sectional view on the line II-II of Fig. l, inthe direction of the arrows.

spacing of the turns of said coils is effected by fiattening, orformingthe tubing oval rather than circular in section.

In melting metals, particularly in melting the more reactivehigh-melting-point metals, such as titanium, zirconium, vanadium,tantalum,

pper Montclair, N. J., asuse Electric Corporation, a corporation ofPenn- Serial No. 252,394

molybdenum and other metals of similar chemical and physicalcharacteristics, diiiiculty is usually encountered in the selection ofcrucible materials. When such metals are melted they react with thematerial of the crucible to some extent, or other reactions occur whichadversely affect the quality of the melt. For example, titanium, whenmelted in a graphite crucible, may contain several tenths to 1% or moreof carbon. When zirconium is melted in contact with beryllium oxide, itbecomes embrittled because of absorbing oxygen from the refractory. Whenmolybdenum is melted in a crucible of water-cooled copper, the ingotthereafter formed has a very rough surface and often possesses holes orvoids.

In order to overcome such diiiiculties, I have devised an improvedapparatus wherein such metal is heated and melted out of contact withany solid material with which it might react, whereby no refractorycrucibles are necessary to hold the metal for melting. The inventioncomprises levitating the metal to be melted in air, a vacuum or inertatmosphere confined by a suitable vessel, such as a quartz, or a 96%silica glass bell jar manufactured by the Corning Glass Works under thename Vycor, by means of an alternating magnetic field or fields.

The levitating magnetic fields are so distributed between, for example,two coaxial coils, that the metal to be melted does not touch anyphysical material and, while so levitating the metal, melting it byalternating current energy from the surrounding coil system. Thelevitating magnetic field, which I protect from undesired distortion inaccordance with my invention, is preferably supplied by the same coilsystem which provides the high frequency electrical energy for heatingand melting the levitated material. Besides keeping the metal out ofContact With any material which might react therewith and contaminatethe melt, volatile materials which may vaporize from metal upon cooling.In previous applications using levitation meltties of form which aresuitably designated as wings or pseudopodia These wings, afterformation, tend to induce an instability in the molten mass causing itto move toward the device creating the electromagnetic, field and thisphenomenon ultimately may result in a shortcircuiting of those elementscreating said electromagnetic eld.

It has been observed that the aforementioned wings demonstrated apredictable tendency to incline or direct themselves toward thoseelements of the field-producing device which were unsymmetrical withrespect lto the vertical axis of said device. rIhese unsymmetricalelements were the conductors or media by which the held-producingcurrent must be introduced to the device from a current source and are,therefore, an indispensable evil associated with the method.

To obviate the difficulty two alternatives are acceptable:

(a) The assymmetry-producing current elements may be directed from theirrespective points of origin at the field producing devi-ce radially awayfrom the vertical axis of the coil to such a degree of separation,theoretically infinite-but practically several feet, that anymodification of their direction of travel will not materially affectythe configuration of the alternating electromagnetic field in thereaction space. However the customary practical considerations of spacerequired for such an assembly, especially in vacuum technology, makethis device unsuitable for most applications.

(b) The second device consists of placing, at certain suitable sitesadjacent the customary device for levitation melting, formed plates ofcopper or other good or better electrically conducting material ofthickness dictated by known formulas depending upon frequency and sodisposed as to shield the metal object, whether molten or solid, withinthe reaction space from those electromagnetic fields or the effectsthereof which are created by unsymmetrical disposition of conductors.Said device will eliminate the troublesome perturbations withinv thereaction space or reduce them to levels of energy that will have anegligible effect upon said reaction eld. The device is illustrated inthe drawing as applied to one of the possible modifications of alevitation producing device.

Now referring to the drawings illustrating an embodiment of myinvention, 'there is shown a bell jar II formed large enough, if this isconvenient, to enclose coils I2 and I3, which are to be used forlevitating conductive material or metal to be melted or heated. This jarrests on a preferably m-etal base Ill and may be sealed gas orvacuum-tight thereto, preferably by means of suitable wax I5.

in the present embodiment, the coils I2 and I3, desirably of copper`tubing (conveniently of 1/4 dia.) flattened to facilitate close coiling,as shown in detail of Figure 3, although drawn unflattened in the otherfigures to avoid drafting diniculties, the lower coil l2 being helicallycoiled into frusto-conical formation, as illustrated. The upper coil i5is formed as a spiral lying in a horizontal plane, instead of beinghelically coiled into frusto-conical formation. Trlowever, suchconstructions are contemplated as correspond with Figures l to i and theother forms disclosed in the Wroughton et al. application referred to.The outside diameter of the lower coil I2 is larger than that of uppercoil I5, in order to more effectively stabilize the support of the massI6 of metal or other conductive material to be heated. Alternatingcurrent is supplied to the coils i2 and I3 by means of coaxial lines orcables l'i and i3, respectively extending to one end of each coil, theother ends being supportingly connected to and through the efficientlyconducting base Ill, as indicated at I9 and 2 I.

The enclosure provided by the bell jar II, and the supporting base Iii,may be evacuated through pipe 22 and, if desired, thereafter suppliedwith inert gas thereby. In order to place the mass I5 of metal, hereshown generally spherical in dotted lines as unmelted, into theinfluence of the supporting and heating coils I2 and I3, we -haveprovided a lifting device 23. Said device comprises a cup 2i, desirablyformed of low-loss material or alumina and of a size suicient to holdthe material to be subsequently levitated and heated, connected to theupper end of a rod 25, the lower end of. which is connected to thearmature of a solenoid 2?. The armature reciprocates in anoil-containing cylinder 28, and is provided with one or more groovestherealong, so that the oil in said cylinder leaks thereby duringoperation, exerting a dash-pot action and preventing jerky operation, asin the first embodiment disclosed in the Wroughton et al. applicationreferred to. The cylinder is desirably threadably connect-ed to thelower portion of the base It, as indicated at 5 l, and sealed thereto bymeans of suitable wax 32. The cup 24 may be detachably connected to theupper end of the rod 25 by having a bayonet slot 33 engaging anoutstanding pin 3Q on said rod.

The shielding means of the present embodiment comprises a sheet or plateof copper, or other highly conductive metal 35, and a similar sheet orplate 36. Both of these sheets are desirably cylindrically curved aboutthe axis of the coils l2 and I3, and connected to the base Ill by meansof insulating blocks 37 and 35, secured to said base by bolts or thelike 35 and lil. These blocks 37 and S8 are desirably made of not onlyinsulating but refractory material, such as porcelain, and the plates 35and 3E are connected respectively thereto by means of screws or the like42 and 43.

The plates 35 and 35 are so positioned that they clear or lie beyond thecoils I2 and i3, being however apertured or slotted, as indicated at 44,d5 and 45, to allow the leads or extensions from the coils to passtherethrough while substantially shielding the space between the plates35 and 35 from the magnetic effects of the current in the verticalportions of the coil extensions.

The upper edges of the slots i5 and 56 may be subsequently closed byangular caps or clips il and 48, and the lower end of the slot 45 may beclosed by a supporting or stern member 49, secured to the plate 35 bybrazing or other desired means, the lower end of which member 49receives the supporting screws 52. In a similar way, the lower portionof the plate 36 may have depending therefrom a stem or supportingportion 5I, similarly secured thereto, and the lower `end of whichreceives the connecting screws 53.

In order to effect heating or melting of the material, using theapparatus of Figures l and 2, material l5 is placed in the cup 2li whilethe bell jar II is removed. This may be done eitherv when the cup iselevated, as upon energization of the solenoid 2?, or when the cup islowered, as shown in Figure l upon deenergization thereof. After thishas been accomplished the bell jar may be placed in position, as shownin Figure 1,

and sealed to the base by the wax I5. The space inside the jar I I, maythen be evacuated through pipe 22, and said vacuum either maintained orthe exhausted air replaced by inert gas, such as argon, nitrogen, orother gas not reactive with the material to heating and melting theconductive material I5. The frequency is such that the resultant skindepth in the material is a small fraction of the dimension of thematerial.

factor may be adjusted The coil-condenser combination may be poweredfrom a 50 kva., 10,000 cycle generator. The voltage on the leads to thegenerator is substantially the voltage across the coils, but the currentreadings in the line will be smaller than the circulating current in thecoils, because of the resonance effect of the parallel condenser coilcombination.

With a preferred spacing between the coils I2 and I3, of about 11A or aparticular case and generator output readings of about 200 volts, 25amps., a piece of aluminum roughly spherical, about 11A" in diameter,and weighing about 60 grams, was raised to the position shown in Figure2, and remained suspended as illustrated, supported by the highfrequency eld and without solid support or away from said equilibriumposition.

With the metal suspended as indicated in Figoperation of the systemcaused rent which provided the supporting field also serving to heat thepiece. After a few minutes, with the power setting noted, the metalstarted to melt. As it melted, it assumed and maintained the shape likea top, illustrated in full lines in Figure 1, with the tip of the moltenmatter reaching downward toward the center of the lower turn of thelower coil. When it is clesired to remove the molten to drain the moltenAs an alternative, the eld strength may be reduced merely enough toallow the metal to solidify, while still supported by the field. Also,the field may be reduced quickly so as to permit the molten metal todrop as a whole.

Although it is desirable to have an upper coil as well as a lower coilfor supporting the metal prior to and after melting, yet it is possibleto support it and melt a mass of metal with a single lower supportingcoil, like that indicated at l2 in Figure l, provided said coil isshaped frustoconical as there shown or when other alternatives,discussed in the Wroughton et al. application referred to, are employed.In such a case, the metal when molten drops slightly from the levitatedposition when solid.

With an arrangement such as shown in Figure l, we used 7 turns of 1/4outside diameter copper tubing in each of the coils I3 and I2 and cooledthem by water. The conical angle subtended by the turns of the coil I2was 120, and the distance between the plane of the coil I3 and that ofthe lower turn of the coil I2 was 11/2, with a 1 opening in the centersof both of said coils. Using an aluminum sphere 1" in diameter weighting21.5 grams, the levitating circulating current necessary was 630 amps.at a frequency of 10 kilocycles/sec., when the currents in the two coilswere in opposition. The stability of levitation of both solid and moltenmetal with this coil combination is better, as compared with that inwhich the coil I 2 is formed as a flat spiral like theupper coil.

With generally circular, although slightly spiral, coils I2 and I3arranged as in Figure 1 of the Wroughton et al. application referred to,using 7 turns of 1A outside diameter watercooled copper tubing for bothof said coils, with the conical angle the distance between planes of thecoils l l/g", and 1" openings through the lower turns of each coil, asolid 1 diameter aluminum sphere weighing 215 grams was supported whencirculating current of 426 amps. at a frequency of l0 kilocycles/sec.was passed input to the circuit and provided good stable levitation ofboth solid and molten metal.

Although in all forms, the coils are, for convenience, shown generallycircular or spiral in plan, or cylindrically helical, I contemplateother forms, such as those generally square or otherwise polygonal inplan, or prismatically helical.

Although a conductive sphere was considered for simplicity, otherconductive forms may be used with eiectiveness. The previous commentshold for magnetic solids as well as non-magnetic solid or moltenobjects. The inherent agitation of the molten mattei' of the levitationobject is due to the thermal and electrical forces therein. Althoughfrom 20 to 550 grams of metals such as copper, brass, tin, and pressedtitanium powder were successfully levitated and successfully levitatedand heated to the desired degree.

to the 'levitated object. It willbe understood that I contemplate in allforms `of my invention, where desired, .effect-ing melting vin aprotective atmosphere or vacuum, either .the manner disclosed in Figuresl and 2 vor Ain any other desirable way which wil-l occur lto thoseskilled in the art,

,It is also Awithin the province of this invention to include someinstances where heating and melting may be eiected by coils, similarlyshielded as in the illustrated embodiment but Separate from those whichcreate the levitation and with electrical power of frequency the same ordiierent from that producing said field. The heating may take place in4the vatmosphere withont protection .against oxidation, as when meltingmaterial which is either not readily oxidized or `in which oxidation isnot undesirable. It is also understood that in all forms the .supportingand/or heating coils .are desirably cooled by passing watertherethrough.

Although preferred embodiments of my invention have been disclosed, itwill be understood that modifications may be made within the spirit andyscope of the appended claims.

:I claim:

l. Apparatus for levitating, heating and melting .electrically.conductive materials, comprising a coil having a substantially verticalaxis, means comprising leads yto and extending beyond said coil forsupplying alternating current to generate a levitating magnetic field,means shielding the space within said coil from the electromagneticeffects of current in the leads beyond said coil, and means for moving aconductive object to a position on said axis, above said coil, and inthe inuence of said eld, whereby it may be levitated while heated to thedesired extent and held out of contact with any physical supportingmeans.

2. Apparatus for levitating, heating, and melting electricallyconductive materials, comprising a plurality of isolated coils with acommon axis, means comprising leads to and extending beyond said coilsfor supplying alternating current to generate opposing levitatingmagnetic elds therebetween, means shielding the space within said coilsfrom the electromagnetic effects of current in the leads beyond saidcoils, and means for moving said object to a position between saidcoils, on their axis, and in the inuence of said fields, whereby it maybe levitated while heated to the desired extent and held out of contactwith any physical supporting means.

3. Apparatus for levitating, heating, and melting electricallyconductive materials, comprising a plurality of isolated coaxial coilseach with a substantially vertical axis, means comprising leads to andextending beyond said coils for supplying high frequency power togenerate opposing magnetic elds therebetween, whereby an electricallyconductive object' may be levitated in the so-produced alternatingfields, means shielding the space within said coils from theelectromagnetic eifects of current in the leads beyond said coils, meansproviding a protective atmosphere around said object, and means formoving said object into the influence of said fields, whereby it may belevitated between said coils at their common axis while being heated tothe desired extent and held out of contact with any physical supportingmeans.

4. Apparatus for levitating, heating, and melting electricallyconductive materials, comprising aconductive tube helically coiled toupwardly rusto-conical form, means rcomprising extensions of Vsaid tubebeyond said coil for supplying alter- -nating current to generate alevitating magnetic field thereabove, :means for internally cooling theturns of said coil and said extensions, means comprising a conductiveplate disposed between each extension and the adjacent portion of 'thecoil to shield the space Vtherewithin from the electromagnetic effec-tsof current yin said `extensions, and means for vmoving a conductivelobject into the influence 4oi said el'd, whereby it may be levitatedwhile heated to the 'desired extent and held out of contact with anyphysical supporting means.

5. Apparatus for levitating, heating, and .melting electricallyconductive materials, comprising a `conductive tube helically rcoiled toupwardly expanding frusto-conical form, a 'similar tube spirally coiledwith its turns lying `in a plane over said first tube and coaxialtherewith, means comprising extensions lon said tubes beyond said coilsfor supplying alternating current in .series to generate a levitatingmagnetic iield therebetween, means 'comprising a conductive platedisposed between each extension and the adjacent portion of the 'coil toshield the space therewithin from the electromagnetic `effects ofcurrent in said extensions, and means for moving a conductive object toa position on the axis of said coils in the influence of said field,whereby it may be levitated while heated to the ydesired extent and heldout of contact with any physical supporting means.

6. Apparatus for levitating, heating, and melting electricallyconductive materials, comprising a conductive tube ilattenedtransversely for close spacing of the turns thereof and helically coiledto upwardly expanding frusto-conical form, a similar tube spir'allycoiled with its turns lying in a plane over said ilrst tube and havingsaid spiral coaxial with the frusto-conical coil, a conductive base,'extensions on said tubes beyond said coils for supplying alternatingcurrent thereto, said extensions first passing radially away from theaxis of said coils and then down and parallel to said axis, two of saidextensions passing through said base and the other two supportinglyengaged by said base for connecting said coils in series to generateopposed levitating magnetic fields therebetween, and means for shieldingthe space between said coils from the effects of current in saidextensions comprising highly-conductive metal in sheet form, cylindricalabout the axis of said coils, and disposed between said coils and saidparallel extensions, said sheet metal being supported on said base frominsulating means upstanding therefrom and connected thereto and providedwith slots through which said radial portions of the extensions pass,means connected to said sheet metal and closing said slots about saidextensions, and means passing through said base, coaxial with saidcoils, for moving a conductive object to a position on the axis oi saidcoils in the inuence of said field, whereby it may be levitated whileheated to the desired extent, said means comprising a cup formed oflow-loss material, a rod passing through said base and on the upper endof which said cup is supported, the lower end of said rod beneath saidbase carrying an armature, a cylinder mounted on the lower surface ofsaid base and in which said rod and its armature are reciprocatinglymounted, a solenoid surrounding said cylinder, and means for energizingand deenergizing said solenoid to effect operation of said cup throughsaid rod, means mounted on said base for enclosing said levitatture insaid base beneath said enclosing means for evacuating the space undersaid enclosing means or supplying a protective atmosphere to said coils.

7. Apparatus for levitating, heating, and melting electricallyconductive ingly engaged by said base for connecting said coils inseries to generate opposed levitating magnetic elds therebetween, andmeans for shielding the space between said coils from the effects ofcurrent in said extensions comprising highlyconductive metal in sheetform, cylindrical about the axis of said coils, and disposed betweensaid coils and said parallel extensions, said sheet metal beingsupported on said base from insulating space under said enclosing meansor supplying a protective atmosphere to said coils.

References Cited in the le of this patent UNITED STATES PATENTS OTHERREFERENCES Okress et al.: Journal of Applied Physics; vol. 23, No. 5;May 1952; pp. 545-552. (Copy in Scientic Library.)

